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Old 17th Nov 2020, 12:31 pm   #1
Malcolm G6ANZ
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Default Sherwood AV amplifier OSD

This is about not reading the manual. I have a Sherwood audio/video receiver amplifier. It was working perfectly except that it wouldn't save the tuner frequency for more than about 30 mins. I could live with this as it only takes a few seconds to rescan to radio 3 and carry on listening. All other functions worked perfectly. I recently became aware that the on screen display that shows what is selected and the volume level was garbled.
I looked at the video information on a scope and all the syncs were present and correct but at the wrong frequency. The line scan was way too slow. Some investigation on the character generator showed it had a 17 Mhz crystal. This ,I thought ,was for generating the sync timings and since these were wrong I assumed the crystal had stopped working on the correct frequency. I ordered a new crystal and duly fitted it. Result ...the same garbled video.
At this point I looked up the OSD in the users guide. Helpfully it stated that if the OSD button is held down for 5 seconds the video standard will be displayed in the readout. I did this and the standard was set to NTSC. The manual went on to state that the standard could be changed by pressing OSD button once when the standard is displayed. I did this and I had a PAL system and correct display. Note to self RTFM !
Now although the OSD was correct it defaulted back to NTSC if the power was off. Obviously the RAM backup supply was not present. I had a long search through the many pages of the circuits to no avail. The internet was of little help until I discovered a single thread about a slightly different amplifier and the same fault. This indicated that the 0.047F capacitor on the display board had died and when it was replaced with a 0.1F memory retention was restored. So I was looking for a 0.047F cap on the display board. Back to the diagrams with some idea of what to look for. Success by the processor supply feed was a 0.047F cap I had missed.
I pulled the display board out and looked for this capacitor, sure enough it was present in two halves. Obviously well past it. I replaced it with 0.1F 5.5v device soldered to the back of the PCB as it was bigger than the original. I also checked the feed diode. The voltage source for the processor is 6V and the cap is rated at 5.5v so there needs to be the 0.6v drop across the diode. The diode was OK
With the board reassembled the RAM memory was held overnight and probably longer.
Moral of the story, remember to read the manual it sometimes helps.
Malcolm
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Old 17th Nov 2020, 5:12 pm   #2
raditechman
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Default Re: Sherwood AV amplifier OSD

I replaced the memory back up capacitor in my Sherwood AV amplifier about 5 years (or more) ago. At the time I telephoned Sherwood in USA as I could not find the capacitor in the amplifier, they were very helpful.

John
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